The present invention relates to pressure measurement devices, and particularly to pressure transmitter systems that respond to pressure at two discrete locations and that communicate with a separate controller over a two-wire link.
Pressure transmitters having a transmitter housing that includes a differential pressure (".DELTA.P") transducer fluidically coupled to two pressure ports in the housing, are known. Such transmitters further include circuitry in the transmitter housing coupled to the transducer and communicating the measured .DELTA.P or a related process variable such as flow or level, to a distant controller over a two-wire link. The controller energizes the circuitry over the two-wire link.
In some applications, such as level measurement, it is desired to measure differential process fluid pressure at locations separated from each other by a distance much greater than the scale size of the transmitter housing. To make such a measurement it is known to attach to the above described .DELTA.P transmitter flexible oil-filled capillary tubes or impulse piping to fluidically transmit the process fluid pressures to the housing pressure ports. However, such arrangements suffer from errors due to differences in height and temperature of the oil-filled capillary tubes. Flow measurements can also be computed using the same architecture. For example, the pressure differential across an orifice plate can be measured with the pressure transmitters and used to calculate flow in accordance with known techniques.
It is also known to provide a separate pressure transmitter at each of the two process fluid measurement locations, and to electrically couple each of the pressure transmitters to a "hydrostatic interface unit" (HIU). The HIU communicates with the distant controller over a two-wire link, and is wholly powered by a separate unit over a different electrical link. The HIU, in turn, electrically powers and communicates with the pressure transmitters, and performs multiple arithmetic operations on the measured pressures. For example, where the pressure transmitters are mounted on a storage tank of process fluid, the HIU can communicate over the two-wire link a 4-20 mA signal indicative of the process fluid density .rho.: ##EQU1## where .DELTA.P is the process fluid pressure difference between the transmitters, g is gravitational acceleration, and z is the (user-programmed) vertical separation of the fluid measurement locations. This system avoids problems associated with oil-filled capillaries external to the transmitter housing, but has disadvantages of its own such as the need to mount additional electronic transmitters proximate the measurement site and the need for a separate power supply for the HIU due in part to the large number of calculations performed by the HIU and the requirement that it powers the transmitters.